1. Field of the Invention
The present invention relates to a method of treating a dust containing lead, zinc, and other substances and exhausted from a melting furnace to recover metallic zinc at a high purity from the dust.
2. Description of the Related Arts
A material containing iron, lead, zinc, and other substances (hereinafter referred to as iron-contained material) such as scrap discarded by a galvanizing factories can be reclaimed by melting it in a melting furnace and then performing vacuum heating treatment. Various types of melting furnaces can be used to melt the iron-contained material. For example, an electric melting furnace 9 having electrodes 91, 92, and 93 is known, as shown in FIG. 2. In melting an iron-contained material 8 by using the melting furnace 9, unburnt CO gas is subjected to secondary combustion by introducing air thereinto from openings 96 and 97 thereof to increase the melting efficiency. A dust 98 is exhausted from the iron-contained material 8 when it is melted. The exhausted dust 98 is collected by a dust collector 2.
Because the iron-contained material 8 consists of pressed scraps of galvanized sheets, the exhausted dust 98 contains iron, zinc, lead, and other elements.
Table 1 shows an example of a result obtained by analyzing the contents of the exhausted dust 98. The exhausted dust 98 contains 35% of iron Fe, 18.2% of zinc (Zn), and other elements. Most of the iron was present in the form of ferric oxide (Fe.sub.2 O.sub.3) and almost all of the zinc was present in the form of zinc oxide (ZnO).
In Table 1 and subsequent Tables 2 to 4, the term "T.Fe" represents whole amount of Fe contained in the exhausted dust and the term "M.Fe" represents the amount of metallic iron contained in the exhausted dust.
TABLE 1 __________________________________________________________________________ Substances of exhausted dust generated by conventional method Substances T.Fe M.Fe FeO Fe.sub.2 O.sub.3 Zn Mn Pb Al Cl F O __________________________________________________________________________ wt. % 35 2 0 50.2 18.2 2.76 2.48 0.38 4.56 0.01 bal. __________________________________________________________________________ T.Fe: Whole amount of Fe M.Fe: Amount of metallic iron (This applies correspondingly to Tables 2, 3 and 4)
Because the exhausted dust 98 contains a large amount of the zinc oxide as described above, it is difficult to reclaim the iron-contained material 8 from the exhausted dust 98. The reduction of the zinc oxide to metal allows not only the exhausted dust 98 but also the metallic zinc to be reclaimed effectively.
Methods of heating the exhausted dust in vacuum, together with a reducing agent have been proposed to recover the metallic zinc from the exhausted dust 98.
In the conventional vacuum heating method, it is necessary to carry out a process of keeping the addition of the reducing agent to the exhausted dust. Because the reducing agent-adding process involves a complicated procedure, there is a demand for the development of an improved method of treating the exhausted dust.